Computerized tomography (CT) has developed dramatically in recent years, and X-ray CT apparata in particular are rapidly taking their place as one of the most important medical diagnostic aids.
In these X-ray CT apparata, X-rays emitted from an X-ray tube permeate the subject of examination (generally the patient), and are converted into electric charges and thence into images by an X-ray detector including segments of detection elements.
However, the spatial resolution in X-ray CT apparata of this sort depends on the distance between adjacent detection elements in the detector. Thus, spatial resolution improves if both the size of the individual detection elements and the distance between them are minimized. Nevertheless, there is a limit to the extent to which the size of the detection elements can be reduced.
There is a method known as the quarter-quarter (QQ) method whereby spatial resolution can be improved without altering the size of the detection elements, which is to say without altering the distance between them. This is achieved by staggering the alignment-centered axis and the rotation-centered projection axis of the detector by a specified distance. It should be pointed out that what is called the rotation-centered axis runs through the rotational center of the detector and is roughly perpendicular to its rotational plane, while the detector rotation-centered projection axis is a line projecting the rotation-centered axis from the X-ray focal point along the detector. Meanwhile, the alignment-centered axis of the detector is such that the number of detection elements on the detector is the same in relation to that axis, which is roughly parallel to the rotation-centered axis.
To explain the QQ method in greater detail, the alignment-centered axis of the detector is staggered approximately ¼ the width of the detection element in relation to the rotation-centered projection axis. Thus, by making use of projection data detected at the prescribed angle (hereinafter referred to as ‘principal data’) and projection data detected when the detector is rotated approximately 180° from the prescribed angle (hereinafter referred to as ‘QQ data’), it is possible to improve spatial resolution.
FIG. 1(a) illustrates the arrangement of detection elements on a detector used in the QQ method. For the purpose of this description it will be assumed that the detector has detection elements arranged one segment by four channels. It should be noted that by channel direction (CH direction) is meant the alignment of detection elements in a direction which is roughly perpendicular to the alignment-centered axis, while segment direction means a direction which is roughly parallel to the alignment-centered axis. The size of the detector in either direction is referred to respectively as channel width (CH width) and segment width.
Because the rotation-centered projection axis C′ (denoted by a dotted line) of the detector is staggered by approximately ¼ CH width in the channel direction in relation to the alignment-centered axis C (denoted by an unbroken line), what is detected is projection data (QQ data) staggered by approximately ½ CH width in the channel direction in relation to the projection data (principal data) shown in FIG. 1(a), which is the standard. It should be noted that FIGS. 17(a) and (b) are as viewed from the same direction.
Since this QQ data is valuable as data from between the detection elements of principal data, combining (inserting) these two types of projection data makes it possible to obtain data with a resolution of approximately ½ channel width, as represented schematically in FIG. 1(c). This means that theoretically the spatial resolution improves roughly twofold as compared with when the QQ method is not applied.
A further development on the QQ method is represented by the technique described in Japanese Laid-Open Patent Application H7[1995]-84052. This technique involves staggering each detection element segment in the channel direction in a detector in which detection elements of two or four segments are aligned, thus allowing data to be collected simultaneously without rotating the data corresponding to QQ data through 180° with the principal data.
This technique again makes it possible to obtain a resolution which is theoretically twice that of a diagnostic image obtained without using the QQ method.
Yet another technique is one described in Japanese Laid-Open Patent Application H5[1994]-169911, according to which the detector has a three-segment detection element with each segment staggered by approximately ⅓ CH width in the channel direction. Moreover, the rotation-centered projection axis of the detector is staggered by approximately 1/12 CH in relation to the alignment-centered axis.
By making use of principal data in the three segments and QQ data corresponding to that principal data, this technique makes it possible to obtain a resolution which is theoretically some six times that of a diagnostic image obtained without using the QQ method.
Nevertheless, although by using principal data and QQ data the first and second conventional examples described above make it possible to obtain a resolution which is approximately twice that obtained without using the QQ method, there have been limitations to the extent to which this could be further improved. This means, for instance, that in clinical use it is difficult to observe diseases of the temporal bone in the field of otorhinology, diseases of the lung or fine blood vessels in the head (0.3 m), and in particular to distinguish between inflammation and cancer at the lobular level.
Moreover, while the third example cited above reportedly makes it possible to achieve approximately six times the resolution as compared with when the QQ method is not applied, there is no mention of how projection data for a prescribed slice surface is interpolated from the projection data obtained.
Apart from this there are no example of conventional methods in which detectors are provided with X-ray blocking members. X-ray blocking members serve to inhibit the X-ray transmission coefficient within a certain range of X-rays incident on the detection elements.
For instance, in the third example quoted above the images are reconstructed using X-rays detected in ⅙ the channel width of the detection elements. In other words, projection data is acquired with the remaining ⅚ of the channel width overlapping.
When there is a great deal of overlapping projection data, this leads to an indistinctness of image which in turn results in poorer resolution.